Mastectomy rates in the U.S. are increasing, driven by more breast cancer patients opting for a mastectomy with breast reconstruction. The cosmetic and psychosocial benefits to post-mastectomy breast reconstruction may come at a significant functional cost given the invasiveness of these procedures to the musculoskeletal system. It is critical that we understand the functional impact of different breast reconstruction approaches to help us enhance the availability of reconstruction surgeries within minimal functional impact and to better identify patients who will need therapeutic interventions as no standard of post-operative care currently exists. Our long term goal is to alleviate functional deficits following breast reconstruction by improving existing surgical approaches to lessen functional morbidity and enhancing the early detection of patients predisposed to shoulder morbidity. The overall objectives of this proposal is to begin addressing our goal by using ultrasound shear wave elastography (SWE) to identify reconstruction surgeries that significantly impact the pectoralis major, and by determining if these SWE measures can predict post-operative functional deficits. Our central hypothesis is that reconstructive procedures that require muscle disinsertion produce greater mechanical deficits of the pectoralis major and poorer upper extremity functional outcomes. The rationale for our proposed study is that it will allow us to identify the precursors of functional deficits in breast reconstruction patients. We will test our hypothesis by pursing two specific aims: 1) Determine how variations in surgical approaches to immediate or delayed breast reconstruction impact the mechanical integrity of the pectoralis major; and 2) Identify how post-operative mechanical changes to the pectoralis major relate to upper extremity functional status and quality of life. Under the first aim, breast cancer patients undergoing four different approaches to breast reconstruction will have the stiffness of the pectoralis major muscle prospectively evaluated with SWE. Under the second aim, these prospective evaluations of the pectoralis major will be associated with performance-based measures of upper extremity functional status and validated patient reported outcomes. The approach is innovative, in the applicant's opinion, because it presents a new and substantive method for evaluating post-operative function in patients following breast reconstruction, and is an important first step in validating this technology as a prognostic biomarker for future shoulder morbidity. The proposed research is significant, because it is expected to have broad translational importance for the prevention and treatment of shoulder morbidity in breast reconstruction patients. We expect these findings will immediately impacting clinical practice, including the adoption of surgical approaches with significantly reduced functional morbidity and bringing new focus to the for active surveillance in reconstruction patients whose procedures predispose them to functional morbidity. Ultimately, such knowledge has the potential to inform the development of a new standard of care for breast cancer patients in the U.S.